A polyimide resin is excellent in heat resistance and electric insulation, so that it has various applications, for example, printed wiring board, heat resistant adhesive tape, electrical parts, protective layer for semiconductor devices, and interlayer dielectric film. However, the polyimide resin dissolves in a limited number of organic solvents, causing difficulty in handling in some applications. For easy handling, a polyamic acid, which is a polyimide precursor and more soluble in various kinds of organic solvents than a polyimide resin, is applied on a substrate and then heated at a high temperature for a long period of time to be imidized by dehydration and cyclization into a polyimide. This imidization of the polyamic acid to a polyimide requires a long time of heating at a high temperature, which tends to degrade the substrate. Insufficient heating, on the other hand, causes a portion of the polyamic acid to remain, which degrades moisture resistance and corrosion resistance of the polyimide.
In place of the polyamic acid, the invention described in Japanese Patent Application Laid-open No. H02-36232 employs an organic-solvent soluble polyimide resin, and a polyimide resin film is made by applying a solution of the organic-solvent soluble polyimide resin on a substrate and heating it to evaporate the solvent. However, the film obtained from the polyimide resin does not have good solvent resistance.
Japanese Patent Application Laid-open No. H10-195278 describes a heat-curable composition which has improved solvent resistance by comprising a polyimide silicone resin having a phenolic OH group and an epoxy resin. However, the polyimide silicone resin is not sufficiently solubility in a solvent. Further, a cured product thereof does not have desired flexibility and tends to be peeled off from a substrate by heating or humidity. To improve the solubility and the flexibility, one may increase an amount of silicone moieties in a polyimide silicone resin. However, such a resin, in turn, has less phenolic OH group, leading to poorer solvent resistance of a cured product therefrom.